1. Field of the Invention
The present invention relates to a receiver that attenuates a disturbance signal such as an image signal which causes disturbance in signal reception in radio equipment, and a disturbance signal attenuating method that is used in such equipment.
2. Description of the Background Art
Heretofore, in receivers in radio equipment, a radio frequency (RF) signal has been converted to an intermediate frequency (IF) signal by a frequency mixer such as an image rejection mixer. In addition, in conventional receivers, in the case where the frequency of the IF signal is high, in order to suppress a disturbance signal such as an image signal which causes disturbance in signal reception, a band-pass filter (BPF) is provided in the stage before the image rejection mixer for passing only a predetermined frequency band therethrough, thereby an image signal in an RF signal being attenuated.
For example, in a multi-channel FM receiver disclosed in U.S. Pat. No. 5,517,685 to Aoyama et al., there is provided a variable frequency band-pass filter that selects a frequency-modulated (FM) signal on a desired channel in response to a channel selection command signal issued from a controller, and only a desired channel is selected, whereby image disturbance can be suppressed.
However, in the multi-channel FM receiver disclosed in Aoyama et al., in the case where the number of channels increases, the variable frequency range of the variable frequency band-pass filter becomes wide and thus it is difficult to implement this receiver.
In addition, in the conventional receiver, if the frequency of an IF signal becomes low, the band-pass filter passes a disturbance signal as it is and therefore the attenuation of the disturbance signal becomes slight. Thus, it is difficult to meet reception standards for radio equipment.